feat: Add QR code generator for Wi-Fi and URLs

This commit introduces a `QrCodeGenerator` utility object to create QR code bitmaps for both Wi-Fi credentials and URLs.

Key features:
- **`generateWifiQr`**: Creates a QR code using the standard `WIFI:` format, allowing other devices to connect to a hotspot by scanning the code. It properly escapes special characters in the SSID and password.
- **`generateUrlQr`**: Generates a standard QR code for any given URL.
- **Implementation**: Uses the `zxing` library to encode the data and converts the resulting `BitMatrix` into an Android `Bitmap`.

app/src/main/java/com/bitchat/android/hotspot/QrCodeGenerator.kt

@@ -0,0 +1,126 @@
+package com.bitchat.android.hotspot
+
+import android.graphics.Bitmap
+import android.util.Log
+import androidx.core.graphics.createBitmap
+import androidx.core.graphics.set
+import com.google.zxing.BarcodeFormat
+import com.google.zxing.common.BitMatrix
+import com.google.zxing.qrcode.QRCodeWriter
+
+/**
+ * Utility for generating QR codes for Wi-Fi connection and URL.
+ */
+object QrCodeGenerator {
+
+    private const val TAG = "QrCodeGenerator"
+
+    /**
+     * Generate QR code for Wi-Fi connection.
+     * Format: WIFI:S:{SSID};T:WPA;P:{PASSWORD};;
+     *
+     * This format is recognized by most Android/iOS devices for instant Wi-Fi connection.
+     *
+     * @param ssid Wi-Fi network name
+     * @param password Wi-Fi password
+     * @param sizePx Size of the QR code in pixels
+     * @return Bitmap of the QR code, or null on error
+     */
+    fun generateWifiQr(ssid: String, password: String, sizePx: Int): Bitmap? {
+        if (ssid.isBlank() || password.isBlank()) {
+            Log.w(TAG, "SSID or password is blank")
+            return null
+        }
+
+        // Escape special characters
+        val escapedSsid = escapeWifiString(ssid)
+        val escapedPassword = escapeWifiString(password)
+
+        // Format: WIFI:S:{SSID};T:WPA;P:{PASSWORD};;
+        val wifiString = "WIFI:S:$escapedSsid;T:WPA;P:$escapedPassword;;"
+
+        Log.d(TAG, "Generating Wi-Fi QR code for SSID: $ssid")
+
+        return generateQrBitmap(wifiString, sizePx)
+    }
+
+    /**
+     * Generate QR code for URL.
+     *
+     * @param url Website URL (e.g., "http://192.168.49.1:9999")
+     * @param sizePx Size of the QR code in pixels
+     * @return Bitmap of the QR code, or null on error
+     */
+    fun generateUrlQr(url: String, sizePx: Int): Bitmap? {
+        if (url.isBlank()) {
+            Log.w(TAG, "URL is blank")
+            return null
+        }
+
+        Log.d(TAG, "Generating URL QR code: $url")
+
+        return generateQrBitmap(url, sizePx)
+    }
+
+    /**
+     * Generate QR code bitmap from string data.
+     *
+     * @param data String data to encode
+     * @param sizePx Size of the QR code in pixels
+     * @return Bitmap of the QR code, or null on error
+     */
+    private fun generateQrBitmap(data: String, sizePx: Int): Bitmap? {
+        if (data.isBlank() || sizePx <= 0) {
+            Log.w(TAG, "Invalid data or size: data.length=${data.length}, sizePx=$sizePx")
+            return null
+        }
+
+        return try {
+            val matrix = QRCodeWriter().encode(
+                data,
+                BarcodeFormat.QR_CODE,
+                sizePx,
+                sizePx
+            )
+            bitmapFromMatrix(matrix)
+        } catch (e: Exception) {
+            Log.e(TAG, "Error generating QR code", e)
+            null
+        }
+    }
+
+    /**
+     * Convert BitMatrix to Bitmap.
+     * Pattern from VerificationSheet.kt.
+     */
+    private fun bitmapFromMatrix(matrix: BitMatrix): Bitmap {
+        val width = matrix.width
+        val height = matrix.height
+        val bitmap = createBitmap(width, height)
+
+        for (x in 0 until width) {
+            for (y in 0 until height) {
+                bitmap[x, y] = if (matrix[x, y]) {
+                    android.graphics.Color.BLACK
+                } else {
+                    android.graphics.Color.WHITE
+                }
+            }
+        }
+
+        return bitmap
+    }
+
+    /**
+     * Escape special characters in Wi-Fi SSID/password for QR code format.
+     * Special characters that need escaping: \ ; , " :
+     */
+    private fun escapeWifiString(input: String): String {
+        return input
+            .replace("\\", "\\\\")  // Backslash must be escaped first
+            .replace(";", "\\;")
+            .replace(",", "\\,")
+            .replace("\"", "\\\"")
+            .replace(":", "\\:")
+    }
+}